Scanned-Spot-Array Optical Lithography is a lithographic printing method in which an array of diffraction-limited focused-radiation spots is raster-scanned over a printing surface (a photosensitive optical recording medium) to synthesize a high-resolution recorded image. The spots may be individually modulated by a spatial light modulator. Systems of this type are described in U.S. Pat. No. 6,133,986, “Microlens Scanner for Microlithography and Wide-Field Confocal Microscopy” (hereafter the '986 patent), and in U.S. Pat. No. 6,897,941, “Optical Spot Grid Array Printer” (hereafter the '941 patent). Alternatively, the spots are not individually modulated, but are collectively modulated by a single modulator. A system of this type is described in the '986 patent.
A similar method can be employed in the context of Scanned-Spot-Array Optical Microscopy, wherein an array of focused-radiation illumination spots is raster-scanned over an inspection surface and the radiation reflected from (or transmitted through) the surface at each spot is collected and detected to construct a high-resolution raster image of the surface. Systems of this type are described in the '986 patent and in U.S. Pat. No. 6,639,201, “Spot Grid Array Imaging System” (hereafter the '201 patent).
The advantage of the scanned-spot-array method in the context of lithography is that it can provide high-throughput maskless printing capability, and it also eliminates optical proximity effects. In the context of microscopy, the method provides a capability for massively parallel confocal imaging.
An additional advantage of the method, in the approach described in the '986 patent, is that it can eliminate the need for wide-field, high-NA projection lenses, which account for much of the complexity and expense of conventional lithography and inspection microscopy systems. The systems described in the '986 patent use a comparatively low-NA projection lens in conjunction with an array of high-NA microlenses close to the printing or inspection surface. The microlenses need only achieve good on-axis point-imaging performance; there is no tolerance requirement on field flatness, distortion, or off-axis aberrations. Thus the microlenses should, in principle, be able to achieve imaging performance comparable to or better than state-of-the-art projection lenses. But it can be difficult to achieve this objective in practice because of the difficulty of forming small, high-NA microlenses to the requisite tolerances for diffraction-limited imaging.
By contrast, the systems described in the '941 and '201 patents separate the radiation into individual spots before it enters the projection lens, which images the spot array onto the printing or inspection surface at reduced magnification. (See the '941 patent at column 6, lines 53-56, and the '201 patent at column 7, lines 15-27.) The spots can be generated by a microlens array, but in this context the microlenses would be low-NA elements, and the spots may be filtered by an aperture array at the microlens focal plane. Low-NA microlenses could be fabricated more easily than the high-NA elements used in ' the '986 patent, and furthermore the spatial filtering would relax the microlens design requirements on aberration and scatter. But this approach requires a high-NA projection lens, which must meet stringent tolerance requirements on field flatness, distortion, and aberrations across a wide image field.
Thus, these two design alternatives involve a tradeoff. If the spots are generated by a microlens array close to the printing or inspection surface, then a very simple, low-NA projection lens can be used but the high-NA microlens design requirements are very challenging. If the spots are imaged through the projection optics, then they can be generated with comparatively simple, low-NA microlenses, but then the system requires a wide-field, high-NA projection lens meeting stringent diffraction-limited performance specifications.